Mobile Node Positioning in Mobile Ad Hoc Network

Transcription

1 Moble Node Postonng n Moble Ad Hoc Networ G.Lu, R.M.Edwards, C.Ladas The Unversty of Sheffeld Abstract: Many Ad Hoc networ rotocols and alcatons assume that the locaton of moble nodes can be obtaned by the GPS system. Although ths s usually true, there are cases where the GPS sgnal s not accessble. In ths aer, we roose an mroved eucldean-based algorthm to gve a full soluton for Ad Hoc node ostonng. We am to rovde moble nodes wth hgh recs on GPS coordnates when the GPS sgnal can be artally receved or alternatvely rovde locaton awareness n a local coordnate system f GPS s comletely naccessble. 1 Introducton Moble Ad Hoc NETwors (MANET) consst of wreless moble hosts whch can communcate wth each other n the absence of a fxed nfrastructure. The moblty of Moble Nodes (MN) can easly cause the toology to change and consequently result n frequent udatng of routng tables. In addt on, the lmted bandwdth comared to wred networs s another ey factor that needs to be consdered when nvestgatng acet routng algorthms. Researchers have roosed several aroaches, such as the Ad hoc On Demand Dstance Vector routng (AODV)[1], the Dynamc Source Routng (DSR)[2] etc, to solve the routng related ssues. Among them, the Locaton-Ad Routng (LAR) [3] algorthm and Dstance Routng Effect Algorthm for Moblty (DREAM) [4] dffer n that they can tae advantage of the locaton nformaton of each node n order to reduce the routng area and route dscovery overheads. Both of these rotocols assume all the nodes can receve the Global Postonng System (GPS) sgnal, wth whch ther oston s detected. However ths s not always ossble as GPS does not wor ndoors. MANET s caable of beng constructed anytme and anywhere, and therefore a much more flexble and owerful aroach needs to be taen. An mroved algorthm s necessary that uses locaton nformaton for ts otmsaton regardless of the resence or not of GPS sgnal. Moreover, such nformaton can be used for locaton based alcatons, such as locaton mas, local news, traffc nformaton, etc. Ths aer defnes a new algorthm for the ostonng of MNs, whch can oerate n both GPS-free and GPS-aded envronments. GPS coordnates can be calculated f two or more neghbourng nodes have GPS based coordnates, otherwse a relatve local locaton s rovded nstead, whch s suffcent for the routng algorthms. 2. Moble node ostonng n MANET Our research s based on the followng assumtons: Every node n the networ has the same confguraton,.e. they have the same software and hardware characterstc; Omn-drectonal antenna s used by all nodes wth dentcal hyscal roertes; The velocty of the MN s lmted to certan boundares; Two dmentonal (2-D) coordnates are used, whlst heght and tme values are gnored; the dstance between MNs can be obtaned by some aroach such as Tme Of Arrval (ToA), Tme Dfference of Arrval (TDoA)[5], etc. Addtonally, several terms whch are used n the followng sectons are defned here. Reference Coordnate System (RCS) s the ntal coordnate system created by three selected nodes at ntal tme. Local Coordnate System (LCS) s our target GPS-free coordnate system derved by RCS. Fae -GPS Coordnate System s the GPS-synchronsed coordnate system whch we am to obtan f certan condtons are satsfed. GPS Coordnate System s the real coordnate system created by a functonal GPS system. Offsets of X, Y coordnates are used to shft LCS to the Fae-GPS doman. Rotaton Angle s the angle at whch the LCS must be rotated n order to have dentcal drecton wth the Fae-GPS (or GPS) coordnates. GPS-node s the MN whch can receve the GPS sgnal drectly. TP-node has ts LCS doman locaton and Fae-GPS-node has ts locaton n the Fae-GPS doman. 2.1 Reference Coordnate System (RCS) At the ntal stage, the neghbours of node can be detected by usng HELLO acets. We use N to denote the set of neghbors of node. The dstances between the neghbours and node are calculated and denoted as D. To generate the RCS, we choose node, (one of the two nodes must be a GPS-node for the coordnates converson) wth the dstance between them d? 0, and d +d? d, d +d? d, d +d? d must be satsfed,.e. the,, cannot be located n the same lne. We defne node as the centre of ths coordnate system,.e. the coordnates of node are x =0, y =0. We choose one of the two neghbours ( s chosen here. s GPS-node) to le on the X-axs wth ostve sgn. Fgure 1 shows the generated RCS. We got two nodes locaton by x =0, 2 d y =0; x =d, y =0. In the trangle?, we can use the cosne rule to get the + d d = arccos snce 2dd d, d, d are nown. Therefore, the locaton of node can be calculated easly by x =d cos?, y =d sn?.

2 TP_Y Rotaton Angle: θ TP_Y ` offset_y offset_x θ offset_y TP_Y offset_x Rotaton Angle Fgure 1 RCS Intalzaton Fgure 2 Offset & Rotaton Angle Fgure 3 Y-fl of RCS 2.2 Offset and Rotaton Angle As llustrated n Fgure 2, to convert the LCS to Fae -GPS Coordnates, we are gong to fnd out the offset and rotaton angle the converson needs. Two nodes out of the three ntal nodes must be GPS-node, one for calculatng the offset, and the other for the rotaton angle. A GPS-node s chosen as the centre of the RCS, thus we get offset_x=x -gs,, offset_y=y -gs. Another GPS-node (say ) s used for the rotaton angle calculaton. Accordng to the coordnate rotaton equatons (Equaton 1): X cos sn X = θ θ r (Equaton 1) Y cosθ snθ Y ( ) ( )( ) where X r, Y r are the orgnal coordnate, X, Y are the coordnate after the rotaton,? s the rotaton angle. We get: fae_ gs _ X = ( t _ X * rotaton_cos + t_ Y * rotaton_sn) + offset_ X (Equaton 2) fae_ gs _ Y = ( t_ Y* rotaton_cos t_ Y* rotaton_sn) + offset _ Y the cosne and snusodal value of rotaton angle can be derved by: cos_ value = ( t _ X * delta_ x+ t _ Y * delta _ y)/( delta _ x + delta _ y ) (Equaton 3) sn_ value = ( t _ X * delta _ y + t_ Y* delta_ x)/( delta _ x + delta_ y ) where delta_x=fae_gs_x-offset_x and delta_y=fae_gs_y-offset_y. Fgure 3 shows another ossble drecton of RCS. In ths secal stuaton, the rotaton Equaton 1 cannot be used because t s mossble to rotate such RCS to GPS drecton. Ths ssue can be solved by slng the t_y value of node,.e. we nvert the Y-axs of RCS whch results n node movng to. Then the coordnate system becomes smlar wth Fgure 2. When the RCS s generated, the node should broadcast a short acet to the networ declarng that t s the centre of ths RCS. The acet should contan ts address, the offset, rotaton value and oston udate sequence number. 2.3 Postonng n Neghbour-Area Nodes of Centre node As shown n Fgure 4, node? N,?, for whch all the dstances between any two nodes are nown, then we can use trangulaton to calculate the TP oston of node. x = d cos (Equaton 4) y =± d sn 2 d + d d where? can be obtaned by usng the cosne rule = arccos. The dffculty here s to 2dd obtan the sgn of value y. Consderng the oston of node, the four ossble cases are llustrated n Fgure 4. For case 1 and case 2, we can fnd thata -ß =? or ß -a =?, but for the cases 3 and 4, we geta +ß =? wherea,ß,? can be calculated by trangle the cosne rule. Therefore, the sgn of node can be determned accordng to the relatonsh of these three angles. 2.4 Postonng n Non-Neghbour-Area Nodes of Centre node For non-neghbor-area node q D, the neghbors locatons become the only reference for each node n that we can only obtan the dstance to the neghbours. Several flags are et n each node to ndcate the oston status and must be set after each udate. Each MN mantans a lst of ts neghbours as well as ther oston status n order to now how many GPS-nodes, Fae-GPS-nodes and TP-nodes are around. Equatons 5 calculate the coordnates based on the aroach resented by Fgure 5. Each MN checs the locaton status of the neghbours erodcally and decdes whch r

3 doman t can obtan each tme. If the MN cannot fnd the thrd neghbour s assstance after several rounds, the locaton wth the furthest dstance from the reference centre s chosen. Case 1 Case 2 Case 3 Case Converson from LCS to Fae-GPS Coordnate Fgure 4 Four ossble cases n centre-neghbor area ostonng After the revously descrbed rocedure s erformed, each LCS locaton must be converted to GPS coordnates by usng the offset and rotaton angle whch are broadcasted by the centre node when RCS s ntalzed. The converson equaton s Equaton 2. B C E B B C B C BC D B D B D BD (Equaton 5) Fgure 5 Non-centre-neghbour area ostonng B E B E BE 2.6 Reference Centre Node In ths algorthm, the choce of Reference Centre Node (RCN),.e. the choce and mantenance of node lays a very mortant role whch greatly determnes the effcency of the entre algorthm Reference Centre Node Intalzaton The RCN node must be chosen smartly. The densty of each node (we use the metrc nown as degree of assocaton stablty,.e. the number of neghbours) s measured by countng the receved HELLO messages sent by ts neghbours. Then, each node, broadcasts ts measured densty to the networ. Nodes wth lower densty factor must sto broadcastng once they dscover that there s a neghbour wth hgher densty. The hgher densty factor as well as that neghbour s dentty are stored n each node. In the next HELLO erod, the new densty factor and neghbour dentty are broadcasted nstead. After a few ntervals, only one node remans wth the hghest densty node, whch s declared as the RCN. If more than one nodes have the same hghest densty, one of them s selected randomly to become the RCN. (n the smulaton, the node wth smaller dentty s et) Reference Centre Node Mantenance The new centre must be chosen erodcally n order to adat to the frequently changng toology. Every GPS-node wth at least one neghbourng GPS-node can declare that t wants to be the centre of the system when t fnds that the assocablty exceeds the defned threshold. Effectvely, only one set of assocablty and node dentty can be et by the MNs. After the next udate nterval, the degree of assocaton stablty n each node should be cleared and recounted. The HELLO message contans a sequence number whch ncreases by one after each tme nterval to ndcate the rounds of udate. When the node receves the HELLO message, t checs the sequence number and comares t wth ts own. A larger receved number ndcates that there s newer ostonng nformaton avalable. 3. Smulaton and Results The roosed ostonng algorthm has been smulated usng the Networ Smulator 2 (NS2) tool. For the smulatons a 670m 2 area was used, antenna range 250m, and 100 nodes. It s denoted when dfferent values are used. Here, the obectve s the oston coordnates calculaton, so accuracy s used as a metrc wth hgh sgnfcance. The results are llustrated n Fgure 6. We test the accuracy aganst the roorton of GPS-node (GPS_RATE), numbers of MNs, average velocty of the MNs, ostonng udatng nterval. In Fgure 6a, t can be notced that after the GPS_RATE exceeds the rate of 35%~40%, the accuracy reaches

4 nearly 100%. The smulaton was erformed for three dfferent scenaros, havng dfferent random moblty arameters. In scenaro 3, oston error s of the range of 110m (for GPS_RATE=0.2), smlar to the GPS error (tycally 100m). Ths ustfes the use of our method, even for the worst case scenaro GPS_RATE vs [Node=50] Scenaro1 Scenaro2 Scenaro GPS_RATE [a] Nodes vs GPS_RATE= Number of Nodes [b] GPS_RATE= AverageSeed vs AverageSeed=0.1m/s AverageSeed=1m/s AverageSeed=5m/s GPS_RATE [c] UdateInterval vs [Node=100] AverageSeed=5m/s UdateInterval (sec) [d] AverageSeed=1m/s Fgure 6 Smulaton results llustratng the accuracy of our ostonng algorthm Fgure 6b, demonstrates that as the number of nodes ncreases, the ostonng accuracy ncreases as well. Note that hgher GPS_RATE contrbutes to the overall accuracy. When nodes are reduced to 20 and GPS_RATE = 0.3, the result s not satsfactory. The reason s that fewer nodes wthn the same smulaton area result to lower connectvty. Moreover, Fgure 6c shows that as the MN velocty ncreases the accuracy dros. Ths s attrbuted, to the need to reconfgure the networ more often. Ths adverse effect can be amelorated by reducng the udate nterval. Ths s shown n Fgure 6d, where the accuracy can be mroved u to 17.5%. 4. Concluson. In ths aer, a new ostonng algorthm for moble ad hoc networs s resented. We show that a relatve coordnate system can be roduced by the self-organzaton of MNs wthout the nowledge of the networ toology. The novelty of ths method s that t can oerate wth artal or even comlete absence of GPS nformaton. Two neghbourng GPS-nodes are enough to rovde all nodes n the networ wth hgh accuracy GPS oston. We have smulated ths method, and shown that the accuracy of the calculated coordnate system can reach as hgh as 100%, wth relatvely small number of GPS-nodes. Factors that affect t are the velocty as well as the total number of nodes n the system. In concluson, ths algorthm can rove to be a comlete ostonng soluton for MANET, whch can be mlemented as a relmnary rocedure for routng rotocols and locaton based servces. References. [1] C.E. Perns and E.M.Royer, Ad-hoc On Demand Dstance Vector Routng, n Proceedngs of the 2nd IEEE Worsho on Moble Comutng Systems and Alcatons, New Orleans, LA, Feb.1999, [2] Davd B. Johnson and Davd A. Maltz, Dynamc Source Routng n Ad Hoc Wreless Networs n Moble Comutng, T.Imelnsy and H.Korth, Eds., Kluwer Academc Publshers, 1996 [3] Y.Ko and N.H. Vadya, Locaton-aded routng (LAR) n moble ad hoc networs, n Proceedngs of the ACM/IEEE Internatonal Conference on Moble Comutng and Networng (Mobcom), 1988, [4] S.Basagn, I.Chlamtac, V.R.Syrotu, and B.A.Woodward, A dstance routng effect algorthm for moblty (DREAM), n Proceedngs of the ACM/IEEE Internatonal Conference on Moble Comutng and Networng (Mobcom), 1988,

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